Synchronous Digital Hierarchy (SDH) is a digital communication hierarchy in optical fibre communication systems and an international standard officially recommended by International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in 1988. SDH is both a networking principle and a multiplexing method. Based on SDH, a flexible, reliable, and remotely-controllable nation-wide and even world-wide telecommunication network may be constructed. This network can make it easy to provide new services while making equipment produced by different manufacturers compatible with each other.
There has been no unified international standard for previous optical communication systems, and different countries have developed different systems, respectively, which are referred to as Plesiochronous Digital Hierarchy (PDH). As a result, the transmission rate of signals, the type of line codes, the interface standards and architectures adopted by different countries are different. And it is impossible to implement compatible devices produced by different manufacturers, or direct networking thereof on optical lines, which results in technical problems and increased cost.
SDH emerges to overcome the shortcomings of PDH such that it is possible to configure the systems and devices required by future communication networks in a perfect mode. SDH has the major features as follows:
1. SDH unifies the transmission rate of signals of various levels in the hierarchy worldwide. The rate defined by SDH is N×155.520 Mb/s, where Mb/s represents the Megabits transmitted per second, bit is a measurement of information, and N=1, 4, 16, 64 . . . . The most common transmission formats include STM-1, STM-4, STM-16, and STM-64, corresponding to the transmission rate classes of 155 Mb/s, 622 Mb/s, 2.5 Gb/s, and 10 Gb/s, respectively, where Gb/s represents gigabits transmitted per second.
2. SDH simplifies the multiplexing and de-multiplexing technique. SDH is able to directly multiplex a signal with a rate of 2 Mb/s into a signal with a rate of 140 Mb/s, or directly de-multiplex a signal with a rate of 140 Mb/s without class by class de-multiplexing. Thus the multiplexing and de-multiplexing technique is simplified, which makes it easy for signals of various rates to be loaded or unloaded on the lines and improves the flexibility and reliability of the communication network.
3. SDH defines a worldwide universal standard of optical interface such that devices produced by different manufacturers may inter-work with each other according to the unified interface standard, which saves the cost of the network.
4. In the frame format for transmission, more redundant bits are reserved for management and control of the network, which greatly enhances the network capability of detecting failures and monitoring transmission performance.
SDH is often used in optical fibre communications, and in order to save resources of optical fibre and lower costs, SDH is typically used in combination with a Wavelength Division Multiplexing (WDM) system. In another words, multiple single-wavelength optical signals carrying services are combined via a WDM system for transmission on one optical fibre.
As shown in FIG. 1, in a typical WDM system, multiple signals are received via a plurality of Optical Transponder Units (OTU). Each signal is converted into a single-wavelength optical wave by an OTU and are transferred in the format of SDH or other format; the single-wavelength optical wave after conversion enters a wave multiplexer/demultiplexer from each OTU for wave combination, and is transferred via an optical fibre to the wave multiplexer/demultiplexer of the destination node; the wave multiplexer/demultiplexer of the destination node carries out wave separation for the received multi-wavelength optical signal and acquires multiple single-wavelength optical signals; each acquired single-wavelength optical signal is then converted via an OTU into an electrical signal of SDH or other transmission format and is outputted after processing to the device at the client side. Optical Amplifiers (OA) may be configured on the transmission line to amplify the optical signal.
By this method of multiplexing single-wavelength optical signals into one optical fibre for transmission, the WDM system implements the data transmission with large capacity. That is why the WDM technique is frequently adopted in the construction of backbones of wide-area or metropolitan-area networks. In some large metropolises, the transmission range of a metropolitan-area network will be up to 300 kilometers or so. At present, in the WDM backbone of a metropolitan-area network, the transmission rate class of a single-wavelength optical signal is typically 2.5 Gb/s or 10 Gb/s.
In practical applications, however, the commonly adopted transmission rate of single-wavelength optical signal at present, i.e. 2.5 Gb/s or 10 Gb/s rate class is not an optimal choice for the transmission rate of a metropolitan-area network.
Major reasons leading to this situation include: on one hand, although the chromatic dispersion allowance of a 2.5 Mb/s rate-class optical signal is large, the requirement thereof for Optical Signal Noise Ratio (OSNR) is low, and the transmission range thereof is long, there are still the shortcomings of low rate and low use efficiency of wavelength; on the other hand, although the use efficiency of wavelength of the 10 Mb/s rate-class optical signal is high, the chromatic dispersion allowance thereof is too low and the requirement thereof on OSNR is harsh.
For example, when the standard G.652 optical fibre is used, in terms of the 2.5 Gb/s rate-class signal, the requirement of the receiver for OSNR is about 20 dB and the transmission range limited by chromatic dispersion is about 960 kilometers. In terms of the 10 Gb/s rate-class service, the requirement of the receiver for ONSR is about 26 dB and the transmission range limited by dispersion is about 60 kilometers. It is seen thus that the optical communication system transmitting signals in these two rates are not suitable to metropolitan-area network nodes of which the transmission range is less than 300 kilometers.